This invention relates to the art of thermal heads for thermal recording which are used in various types of printers, plotters, facsimile, recorders and the like as a recording device.
Thermal materials comprising a thermal recording layer on a substrate of a film or the like are used to record images produced in diagnosis by ultrasonic scanning (sonography).
This recording method, also referred to as thermal recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is under review.
As is well known, thermal recording involves the use of a thermal head having a glaze, in which heating elements comprising a heat-generating resistor and electrodes, used for heating a thermal material to record an image are arranged in one direction (main scanning direction) and, with the glaze urged at small pressure against the thermal material, the two members are moved relative to each other in an auxiliary scanning direction perpendicular to the main scanning direction, and energy is applied to the heating elements of the respective pixels in the glaze in accordance with image data to be recorded which were supplied from an image data supply source such as MRI or CT in order to heat the thermal recording layer of the thermal material, thereby performing image recording through color formation.
A protective coating is formed on the surface of the glaze of the thermal head in order to protect the heating elements and the like. Therefore, it is this protective coating that contacts the thermal material during thermal recording and the heat-generating resistor heats the thermal material through this protective coating so as to perform thermal recording.
The protective coating is usually made of wear-resistant ceramics; however, during thermal recording, the surface of the protective coating is heated and kept in sliding contact with the thermal material, so it will gradually wear and deteriorate upon repeated recording.
If the wear of the protective coating progresses, density unevenness will occur on the thermal image or a desired protective strength can not be maintained and, hence, the ability of the coating to protect the heat-generating resistor is impaired to such an extent that the intended image recording is no longer possible (the head has lost its function).
Particularly in the applications such as the aforementioned medical use which require multiple gradation images of high quality, the trend is toward ensuring the desired high image quality by adopting thermal films with highly rigid substrates such as polyester films and also increasing the setting values of recording temperature (energy applied) and of the pressure at which the thermal head is urged against the thermal material. Under these circumstances, as compared with the conventional thermal recording, a greater force and more heat are exerted on the protective coating of the thermal head, making wear and corrosion (or wear due to corrosion) more likely to progress.
With a view to preventing the wear of the protective coating on the thermal head and improving its durability, a number of techniques to improve the performance of the protective coating have been considered. Among others, a carbon-based protective coating (hereinafter referred to as a carbon protective layer) is known as a protective coating excellent in resistance to wear and corrosion.
Thus, Examined Published Japanese Patent Application (KOKOKU) No. 61-53955 discloses a thermal head excellent in wear resistance and response which is obtained by forming a very thin carbon protective layer having a Vickers hardness of 4500 kg/mm.sup.2 or more as the protective coating of the thermal head. Unexamined Published Japanese Patent Application (KOKAI) No. 7-132628 discloses a thermal head which has a two-layered protective coating comprising a lower silicon-based ceramic protective layer and an overlying diamond-like carbon layer, the protective coating having wear and breakage significantly reduced, thereby ensuring that high-quality images can be recorded over an extended period of time.
As shown in FIG. 3, the heating element of the thermal head usually comprises an under-glaze heat-accumulating layer 102 (hereinafter referred to as "heat-accumulating layer") formed on a substrate 100, a heat-generating resistor 104 overlaid on the heat-accumulating layer 102, and a positive electrode layer 106 and a negative electrode layer 108 formed on the substrate 100 and the heat-accumulating layer 102.
The heating element is coated with a protective coating. In the case of the two-layered structure described above, the heating element is coated with a ceramic protective layer 110, which in turn is coated with a carbon protective layer 112.
The substrate 100 is made of a material such as alumina. The substrate 100 has usually fine irregularities that are reflected on the region of the electrode layers neighboring the substrate 100.
The ceramic protective layer 110 and the carbon protective layer 112 formed on the electrode layers 106, 108 are usually formed by film deposition techniques including sputtering and chemical vapor deposition (CVD), but may often have pinholes 114a, b or cracks due to the irregularities of the electrode layers 106, 108.
The carbon protective layer 112 has a high electric conductivity. Thus, if the ceramic protective layer 110 has the pinholes 114a, 114b or cracks, the current for driving the thermal head does not pass through the heat-generating resistor 104 having low electric conductivity, but enters the carbon protective layer 112 through the pinhole 114a in the ceramic protective layer 110 located on the positive electrode layer 106 side, passes therethrough, and reaches the negative electrode layer 108 through the pinhole 114b in the ceramic protective layer 110 located on the negative electrode layer 108 side.
The pinholes 114a, 114b are small holes. Under such a phenomenon, a large amount of energy is applied to a portion of the electrode layers due to concentration of electric charges, which may often damage the electrode layers. The electrode layers (heating elements) are damaged for several dots to such an extent that the thermal head has lost its function.
Even if the electrode layers are not damaged, an excessive current runs without passing through the heat-generating resistor 104, which results in a damage of various devices including IC used for driving the thermal head.